There is significant interest in lithium sulfur (i.e., “Li—S”) batteries as potential portable power sources for their applicability in different areas. These areas include emerging areas, such as electrically powered automobiles and portable electronic devices, and traditional areas, such as car ignition batteries. Li—S batteries offer great promise in terms of cost, safety and capacity, especially compared with lithium ion battery technologies not based on sulfur. For example, elemental sulfur is often used as a source of electroactive sulfur in a Li—S cell of a Li—S battery. The theoretical charge capacity associated with electroactive sulfur in a Li—S cell based on elemental sulfur is about 1,672 mAh/g S. In comparison, a theoretical charge capacity in a lithium ion battery based on a metal oxide is often less than 250 mAh/g metal oxide. For example, the theoretical charge capacity in a lithium ion battery based on the metal oxide species LiFePO4 is 176 mAh/g.
A Li—S battery includes one or more electrochemical voltaic Li—S cells which derive electrical energy from chemical reactions occurring in the cells. A cell includes at least one positive electrode. When a new positive electrode is initially incorporated into a Li—S cell, the electrode includes an amount of sulfur compound incorporated within its structure. The sulfur compound includes potentially electroactive sulfur which can be utilized in operating the cell. A negative electrode in a Li—S cell commonly includes lithium metal. In general, the cell includes a cell solution with one or more solvents and electrolytes. The cell also includes one or more porous separators for separating and electrically isolating the positive electrode from the negative electrode, but permitting diffusion to occur between them in the cell solution. Generally, the positive electrode is coupled to at least one negative electrode in the same cell. The coupling is commonly through a conductive metallic circuit.
Li—S cell configurations also include, but are not limited to, those having a negative electrode which initially does not include lithium metal, but includes another material. Examples of these materials are graphite, silicon-alloy and other metal alloys. Other Li—S cell configurations include those with a positive electrode incorporating a lithiated sulfur compound, such as lithium sulfide (Li2S).
The sulfur chemistry in a Li—S cell involves a related series of sulfur compounds. During a discharge phase in a Li—S cell, lithium is oxidized to form lithium ions. At the same time larger or longer chain sulfur compounds in the cell, such as S8 and Li2S8, are electrochemically reduced and converted to smaller or shorter chain sulfur compounds. In general, the reactions occurring during discharge may be represented by the following theoretical discharging sequence of the electrochemical reduction of elemental sulfur to form lithium polysulfides and lithium sulfide:S8→Li2S8→Li2S6→Li2S4→Li2S3→Li2S2→Li2S
During a charge phase in a Li—S cell, a reverse process occurs. The lithium ions are drawn out of the cell solution. These ions may be plated out of the solution and back to a metallic lithium negative electrode. The reactions may be represented, generally, by the following theoretical charging sequence representing the electrooxidation of the various sulfides to elemental sulfur:Li2S→Li2S2→Li2S3→Li2S4→Li2S6→Li2S8→S8 
A common limitation of previously-developed Li—S cells and batteries is capacity degradation or capacity “fade”. Capacity fade is associated with coulombic efficiency, the fraction or percentage of the electrical charge stored by charging that is recoverable during discharge. It is generally believed that capacity fade and coulombic efficiency are due, in part, to sulfur loss through the formation of certain soluble sulfur compounds which “shuttle” between electrodes in a Li—S cell and react to deposit on the surface of a negative electrode. It is believed that these deposited sulfides can obstruct and otherwise foul the surface of the negative electrode and may also result in sulfur loss from the total electroactive sulfur in the cell. The formation of anode-deposited sulfur compounds involves complex chemistry which is not completely understood.
In addition, low coulombic efficiency is another common limitation of Li—S cells and batteries. A low coulombic efficiency can be accompanied by a high self-discharge rate. It is believed that low coulombic efficiency is also a consequence, in part, of the formation of the soluble sulfur compounds which shuttle between electrodes during charge and discharge processes in a Li—S cell.
Some previously-developed Li—S cells and batteries have utilized high loadings of sulfur compound in their positive electrodes in attempting to address the drawbacks associated with capacity degradation and anode-deposited sulfur compounds. However, simply utilizing a higher loading of sulfur compound presents other difficulties, including a lack of adequate containment for the entire amount of sulfur compound in the high loading. Furthermore, positive electrodes formed using these compositions tend to crack or break. Another difficulty may be due, in part, to the insulating effect of the higher loading of sulfur compound. The insulating effect may contribute to difficulties in realizing the full capacity associated with all the potentially electroactive sulfur in the high loading of sulfur compound in a positive electrode of these previously-developed Li—S cell and batteries.
Conventionally, the lack of adequate containment for a high loading of sulfur compound has been addressed by utilizing higher amounts of binder in compositions incorporated into these positive electrodes. However, a positive electrode incorporating a high binder amount tends to have a lower sulfur utilization which, in turn, lowers the effective maximum discharge capacity of the Li—S cells with these electrodes.
Li—S cells and batteries are desirable based on the high theoretical capacities and high theoretical energy densities of the electroactive sulfur in their positive electrodes. However, attaining the full theoretical capacities and energy densities remains elusive. Furthermore, as mentioned above, the sulfide shuttling phenomena present in Li—S cells (i.e., the movement of polysulfides between the electrodes) can result in relatively low coulombic efficiencies for these electrochemical cells; and this is typically accompanied by undesirably high self-discharge rates. In addition, the concomitant limitations associated with capacity degradation, anode-deposited sulfur compounds and the poor conductivities intrinsic to sulfur compound itself, all of which are associated with previously-developed Li—S cells and batteries, limits the application and commercial acceptance of Li—S batteries as power sources.
Given the foregoing, what are needed are Li—S cells and batteries without the above-identified limitations of previously-developed Li—S cells and batteries.